Recently, the demand for optical large-capacity recording media capable of recording information at higher density grows year by year. In responding to such a demand, the development of magneto-optical memory media and phase-transition type memory media picks up speed. Also, the use of photochromic materials as recording materials is under research and development. An optical memory medium using a photochromic material is usually produced by forming on a glass or plastic transparent substrate an organic thin film including a photochromic material, such as spiropyrane, fulgide and diarylethene derivatives, and by further forming thereon a metal reflection film, such as aluminum and gold if needed.
Usually, when a colorless photochromic material is exposed to shortwave light such as ultraviolet light, it absorbs visible light and appears colored. On the other hand, when the photochromic material is exposed to visible light, it returns to its original colorless state. Such a color change takes places reversibly on irradiation of shortwave light and of visible light.
Recording, reproduction and erasure of information on such an optical memory medium including a photochromic material are performed using such a photochromic reaction. For example, visible light is irradiated on the entire surface of the optical memory medium to make the recording layer colorless. Then, recording of information is executed by irradiating ultraviolet light on the optical memory medium. Portions of the optical memory medium whereupon information has been recorded have color. Therefore, when visible light with a wavelength which does not cause a photochromic reaction is irradiated on the optical memory medium, the information is read out using a contrast between the colored portion and the colorless portion. When the colored portion is exposed to visible light with a wavelength causing a photochromic reaction and returned to the colorless state, erasure of information is carried out.
With this optical memory medium, it is possible to use shortwave light for reproducing information. Since the shortwave light achieves a light beam of a smaller diameter in comparison with visible light, it is preferable to use the shortwave light when reproducing bits which are recorded at higher density.
Photochromic materials having the above-mentioned properties are now under research and development and expected to be new recording materials for the next generation. In particular, diarylethene series materials attract attention as they show good thermal stability of the recorded information or colored state and excellent durability against repeated color changes. There is a report on a diarylethene derivative which is photosensitive to semiconductor laser light having a wavelength of 780 nm (Journal of Organic Synthesis Science Association, Vol. 49, 1991, p. 373).
With the prior art, such a photochromic compound is dispersed in a polymeric binder, including an acrylic resin such as PMMA (polymethyl methacrylate), a polycarbonate resin, and a polystyrene resin and then formed into a recording layer for a photochromic optical memory medium.
However, though the diarylethene series compounds have the above-mentioned advantageous properties, they show no photochromic reaction or show a weak photochromic reaction in the above-mentioned polymeric binders used for various purposes. Thus, there is a great demand for a polymeric binder which can satisfactory induce the advantageous properties of the diarylethene series compounds.
With a recording medium using a diarylethene series compound, even when information is read out by shortwave light with an intensity lower than that of light used for recording, the shortwave light causes a photochromic reaction in an unrecorded portion and the portion appears colored gradually. Accordingly, the contrast between the recorded portion and the unrecorded portion is gradually lessened, and those portions eventually have the same color. As a result, the recorded information is destroyed, causing non-destructive readout to be infeasible.